1. Field of the Invention
Embodiments disclosed herein relate generally to drill bits. In particular, embodiments disclosed herein relate to hybrid drill bits having roller cones or disks and fixed blades.
2. Background Art
Historically, there have been two main types of drill bits used for drilling earth formations, drag bits and roller cone bits. The term “drag bits” refers to those rotary drill bits with no moving elements. Drag bits include those having cutting elements attached to the bit body, which predominantly cut the formation by a shearing action. Roller cone bits include one or more roller cones rotatably mounted to the bit body. These roller cones have a plurality of cutting elements attached thereto that crush, gouge, and scrape rock at the bottom of a hole being drilled.
Typically, bit type may be selected based on the primary nature of the formation to be drilled. However, many formations have mixed characteristics (i.e., the formation may include both hard and soft zones), which may reduce the rate of penetration of a bit (or, alternatively, reduces the life of a selected bit) because the selected bit is not preferred for certain zones. For example, both milled tooth roller cone bits and PDC bits can efficiently drill soft formations, but PDC bits will typically have a rate of penetration several times higher than roller cone bits.
PDC Drill Bits
Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits.
PDC bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities.
An example of a prior art PDC bit having a plurality of cutters with ultra hard working surfaces is shown in FIG. 1. The drill bit 10 includes a bit body 11 having a threaded upper pin end 12 and a cutter face 13. The cutter face 13 typically includes a plurality of ribs or blades 14 arranged about the rotational axis of the drill bit and extending radially outward from the bit body 11. Cutting elements, or cutters, 15 are embedded in the blades 14 at predetermined angular orientations and radial locations relative to a working surface and with a desired back rake angle and side rake angle against a formation to be drilled.
A plurality of orifices 16 are positioned on the bit body 11 in the areas between the blades 14, which may be referred to as “gaps” or “fluid courses.” The orifices 16 are commonly adapted to accept nozzles. The orifices 16 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades 14 for lubricating and cooling the drill bit 10, the blades 14 and the cutters 15. The drilling fluid also cleans and removes the cuttings as the drill bit 10 rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters 15 may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).
Roller Cone Drill Bits
Roller cone drill bits are generally used to drill formations that fail by crushing and gouging as opposed to shearing. Typically, roller cone drill bits are also preferred for heterogeneous formations that initiate vibration in drag bits. Roller cone drill bits include milled tooth bits and insert bits. Milled tooth roller cone bits may be used to dill relatively soft formations, while insert roller cone bits are suitable for medium or hard formations.
Roller cone drill bits typically include a bit body with a threaded pin formed on the upper end of the bit body for connecting to a drill string, and one or more legs extending from the lower end of the bit body. Referring now to FIGS. 2 and 3, a conventional insert roller cone drill bit, generally designated as 20, consists of bit body 21 forming an upper pin end 22 and a cutter end 23 of roller cones 24 that are supported by legs 25 extending from body 21. The threaded pin end 22 is adapted for assembly onto a drill string (not shown) for drilling oil wells or the like. Each of the legs 25 terminate in a shirttail portion 26.
Each of the roller cones 24 typically have a plurality of cutting elements 27 thereon for cutting earth formation as the drill bit 20 is rotated about the longitudinal axis L. FIGS. 2 and 3 show cutting elements 27 pressed within holes formed in the surfaces of the cones 24; however, milled tooth bits have hardfaced steel teeth milled on the outside of the cone 24 instead of carbide inserts. Nozzles 28 in the bit body 21 introduce drilling mud into the space around the roller cones 24 for cooling and carrying away formation chips drilled by the drill bit 20. Drilling fluid is directed within the hollow pin end 22 of the bit 20 to an interior plenum chamber 29 formed by the bit body 21. The fluid is then directed out of the bit through the one or more nozzles 28.
Each leg 25 includes a journal 30 extending downwardly and radially inward towards a center line, or longitudinal axis, L of the bit body 21. A bearing assembly 31 (e.g., roller bearing, ball bearing, etc.) is disposed between the cone 24 and the journal 30. Roller cones 24 are retained on journal 30 by a plurality of balls 32, which are fitted into complementary ball races 33a, 33b in the cone 24 and on the journal 30, respectively, forming a ball race. These balls 32 are inserted through a ball passage 34, which extends through the journal 30 between the ball races 33a, 33b and the exterior of the drill bit 20. A cone 24 is first fitted on the journal 30, and then the balls 32 are inserted through the ball passage 34. The balls 32 carry any thrust loads tending to remove the cone 24 from the journal 30 and thereby retain the cone 24 on the journal 30. The balls 32 are retained in the races by a ball retainer 35 inserted through the ball passage 34 after the balls are in place and welded therein.
Contained within bit body 21 is a grease reservoir system, generally designated as 36. Lubricant passage 37 is provided from a reservoir chamber 38 to ball race surfaces 33a, 33b formed between a cone 24 and a journal 30. The ball bearing surfaces 33a, 33b between the cone 24 and journal 30 are lubricated by a lubricant or grease composition. Lubricant or grease is retained in the bearing structure by a resilient seal 39 between the cone 24 and journal 30.
Hybrid Drill Bits
Both roller cone and PDC bits have their own advantages. Due to the difference in cutting mechanisms and cutting element materials, they are best suited for different drilling conditions. Roller cone bits predominantly use a crushing mechanism in drilling, which gives roller cone bits overall durability and strong cutting ability (particularly when compared to previous bit designs, including disc bits). PDC bits use a shearing mechanism for cutting, which allows higher performance in soft formation drilling than roller cone bits are able to achieve.
Thus, in drilling operations facing mixed formations, using one type of drill bit over the other may not necessarily be adequate for the entire operation. Hybrid drill bits that use a combination of one or more rolling cutters and one or more fixed blades have been proposed in the prior art. However, problems arise during the design of these hybrid bits in trying to combine rolling cutters and fixed blades within a limited amount of space.
Accordingly, there exists a continuing need for developments in drill bits that may provide advantages of both roller cone drill bits and fixed cutter drill bits.